You'll need to place all of the PNGs on one tiled image in Photoshop first, you can do this manually (dropping in each layer to create a UV tile) or use this helpful Photoshop Script which automates the process.

You can import the single images like normal textures. To create an animation from them you would have to write your own script. (quite easy if you don't mind performance issues)
There is no unity feature that does this for you.